Liquid crystal that is present in liquid crystal cells used in display devices such as LCD has a birefringence property. This birefringence property causes the difference in refractive index of light according to the positions in which a user views the liquid crystal display device, and also causes the difference in degree of polarization when linearly polarized light is passed through the liquid crystal molecules. As a result, the light transmittance and color characteristics observed when a display device is viewed from positions other than the front of the display device are different from those when the liquid crystal display device is viewed from its front. Therefore, this is problematic because it is difficult to realize a high-quality image on the liquid crystal display device.
Therefore, optical films (for example, retardation films) are used in LCD to compensate for the retardance in the liquid crystal cells. In this case, these films function to improve properties such as a viewing angle, a contrast ratio and color characteristics by reversely compensating for a change in retardance of light, wherein the change in retardance of light is generated when the light is passed through the liquid crystal cell. For this purpose, the optical films should be designed to optimize optical characteristics.
One of the inherent optical characteristics of the optical films is wavelength dispersion characteristic. The expression “wavelength dispersion characteristic” means the changes in retardance according to the wavelengths of light when light is incident on an optical film at a given incidence angle and a reference wavelength. In general, the wavelength dispersion characteristics of the retardation films are divided into a normal dispersion, a flat dispersion and an inverse dispersion, depending on the shapes of the dispersions, as shown in FIG. 1.
The retardance value is represented by R(λ)=Δn(λ)×d, wherein Δn represents birefringence, and d represents thickness of a film. From the equation, it might be seen that the retardance value is varied according to the birefringence Δn and the birefringence Δn is a function of the wavelength (λ). As a result, the optical films have wavelength dispersion characteristics that the retardance value of the retardation film is varied according to the wavelength. Therefore, it is possible to control the wavelength dispersion characteristics of the optical films under the control of birefringence.
U.S. Pat. No. 6,800,697 discloses a method for controlling the wavelength dispersion of a retardation film by adjusting a mixing ratio of at least two kinds of copolymers to control the birefringence of materials from which the retardation film is produced.
However, materials that may be used in the manufacture of the retardation film are restricted. Moreover, most of the materials have problems in that phase separation may take place when they are mixed with other materials since they have low compatibility to the other materials. Therefore, the method of controlling the wavelength dispersion characteristics using the materials as disclosed in the U.S. patent has its limits in that the method is performed within a highly restrictive extent.
As an alternative to control wavelength dispersion characteristics of a retardation film, Japanese Patent Laid-Open Publication No. H2-609139 discloses a method for controlling wavelength dispersion characteristics of a retardation film by attaching a plurality of retardation films at a constant angle, the retardation films having different retardance. However, the method also has problems in that it is difficult to attach a plurality of the retardation films to each other at a constant angle, and the transmittance is deteriorated due to the increased thickness of the retardation films.